In many urban settings, it is not possible to install underground optical communications fiber cable using open trenches. Streets, buildings, and underground utilities and infrastructure interfere with digging trenches from the surface. Similarly, when routing utilities such as fiber cables across environmentally sensitive areas such as streams, beaches and wetlands, it is undesirable to excavate a trench. In those cases, and increasingly in more traditional cases, a horizontal directional boring technique may be utilized.
In a horizontal directional boring process, ductwork is placed underground by boring a passage using 20 foot lengths of steel pipe that are fastened together and fed as the passage is bored. A pressurized slurry of clay or another substance is pumped through the steel pipe. The slurry removes material ahead of the pipe, allowing the pipe to be fed through the soil substrate. Additional lengths of pipe are added as the passageway progresses underground. An oscillator or another locator device at the end of the pipe transmits a location signal allowing the end of the pipe to be tracked from above-ground. The direction that the passage takes underground can be controlled as it is formed by aiming the slurry stream at the end of the pipes.
After the underground passage has been bored, a duct is placed in the passage for containment and protection of the optical fibers. The duct may be placed inside the steel pipe before the pipe is removed, or may be fed in from the opposite end and pushed against end of the steel pipe as the pipe is removed.
The ducts are continuous high density polyethylene (HDPE) tubes, which may range in diameter from 1½ inches to 8 inches. The tubes typically have a ⅛ inch wall. Horizontal boring operations have been successfully used to install underground polyethylene ducts thousands of feet in length.
After the polyethylene duct is installed underground, an optical fiber cable is drawn through the duct. Compressed air is often used to perform that operation. The compressed air may act on the fiber optic cable itself, or may be used to place a draw line in the duct for pulling the fiber optic cable through the duct. The compressed air may act on a moveable baffle attached to an end of the fiber cable or draw line.
Problems may be encountered in installing the fiber optic cable in the underground duct. One frequently encountered problem, as shown in FIG. 1, is the collapse of a section 150 of the duct 110 The collapsed duct may be detected as unusually high back pressure when applying compressed air to the duct, or may be a mechanical blockage preventing the cable from passing through the duct.
The collapse is a deformation of the polyethylene wall of the duct 110. The collapse may be caused, for example, by a large rock 120 in the soil substrate 140 that has shifted and applied a concentrated force on the wall of the duct. A collapsed section of duct is a major setback in a horizontal directional boring operation, because the fiber cable or other utility cannot be installed in the duct.
Two options are commonly available to overcome a collapsed duct problem and successfully complete a horizontal directional boring operation. First, the collapsed section may be approximately located using a probe, and then the collapsed section may then be excavated from the surface 130 and repaired. Excavation may be very expensive depending on the location of the problem. More importantly, it is often the case that excavation is not possible for the very reasons that horizontal directional boring was chosen as the installation method. For example, there may be buildings, streets or other utilities above the collapsed section, or the collapsed section may be under an environmentally sensitive area, or under a body of water.
If it is not possible or feasible to excavate the site of the collapsed duct, the other option available today is to re-install the duct by performing another horizontal directional boring operation. That is obviously an expensive last resort, but may be necessary in the absence of other options.
There is therefore presently a need for a reliable method and apparatus for repairing the collapsed section of underground duct without the necessity of excavating the site of the collapse. Such a technique should be relatively low cost, and should be easily used by an online service provider (OSP) technician. To the inventors' knowledge, there is currently no such apparatus or method employed to satisfactorily accomplish that task.